Did You Know? Safety Grounding Mats Don’t Usually Make Great Grounds
A common misunderstanding in the cathodic protection industry is that safety grounding mats are a significant part of an AC mitigation grounding system. The truth is, they may not be very effective as grounds, however they do serve a very important purpose on an AC mitigation system. Let’s take a closer look.
First, AC mitigation systems are needed when pipelines share a common right-of-way with transmission power lines, to alleviate the induced AC effects upon the pipeline from the power line. Such mitigation systems are designed by consultants, using Dairyland decouplers connected between the pipeline and a low resistance grounding system. Grounding mats are often part of the overall mitigation package and needed for personnel protection.
Next, there are three basic terms to understand: grounding (or gradient control) mats, touch voltage, and step voltage. Grounding mats are voltage-limiting wire systems (hence, gradient control) that are intended to manage voltage that could reach harmful levels if not addressed. Touch voltage is the voltage that could occur between a person’s two hands, or between their hands and feet, when they’re standing and touching an electrified object like a pipe. Step voltage is the voltage difference between a person’s two feet if there was a voltage gradient in the earth and the individual was not touching any structure.
Gradient control mats are intended to address AC faults and lighting conditions to reduce the risk of shock or electrocution. And they can, but this is highly dependent upon the design. These mats are typically installed in the earth around piping and test stations, and commonly used on a right-of-way near power lines to address electrical effects from this close proximity. The mat is electrically bonded to the pipe, often through a decoupler. Usually, there is a high resistivity fill, such as gravel, installed over top of the grounding mat.
The goal is to have the grounding mat limit the voltage gradient in the earth and limit the touch voltage. After applying an appropriate mat design, these voltages can be limited. Be warned, however, as some mat designs do not adequately address lightning effects.
For any mat design, to make the grounding mat even more effective, one would add high resistivity fill (gravel) on top of the mat. High resistivity fill is intended to help limit how much current could flow through a person, due to step voltage or touch voltage, and should provide even better effects against electrical conduction than if native soil was used.
Unfortunately, grounding mats are placed at shallow depths where the soil is usually dry, and combined with the high resistivity fill yields a high resistance to earth. If there is high resistance to earth, little AC mitigation will take place. In other words: gradient control mats are not low resistance grounds that provide the needed AC mitigation.
HIGH RESISTIVITY ⇒ HIGH RESISTANCE TO EARTH ⇒ LITTLE AC MITIGATION
Grounding mats will contribute some beneficial grounding to an AC mitigation scheme, but users should not expect large effects. A separate AC mitigation grounding system that is custom designed for a given site is still needed, to achieve the necessary low resistance to earth. However, gradient control mats DO serve a needed purpose for step and touch voltage protection on AC mitigation systems. Any additional grounding effect that comes from grounding mat use is still a benefit to the AC mitigation system.
Grounding mats are often connected to the pipeline via a Dairyland decoupler, to allow full AC conductivity while blocking cathodic protection current exchange. This prevents the mat material from interacting with the pipeline cathodic protection system, but works in conjunction with the mat to provide AC fault and lightning exposure.